This invention relates generally to scanned display systems such as those utilizing a cathode ray tube display device. It relates especially to television receivers which employ velocity modulation of a cathode ray tube's electron beam for image enhancement.
Typically, scan display systems of the type to which the present invention pertains comprise a viewing screen which is scanned by a modulated electron beam to develop an image. The intensity of the displayed image is instantaneously controlled by an intensity control signal. When an abrupt change in brightness level is sought to be reproduced, a problem arises due to the finite time interval required for the intensity control system to effect a change of image brightness level. Because scan continues during this interval, the brightness change in the transition area is gradual, rather than abrupt, producing a blurred edge on the depicted image.
In a cathode ray tube (CRT), for example of the type used as the display device in television receivers, electronic games and computer readout terminals, one or more electron beams are directed toward a phosphor viewing screen which emits light when bombarded by high-energy electrons. The electron beam or beams are caused to scan in both horizontal and vertical directions by an electromagnetic deflection yoke positioned on the CRT adjacent the electron beam path. Control electrodes within the CRT regulate the CRT beam current which, in turn, control image brightness. The limited bandwidth of the system gives rise to the above-described edge blurring when an abrupt or instantaneous change in brightness level is sought.
Television designers have tried various approaches to improve edge sharpness in video display, such as velocity modulation systems which temporarily arrest or slow horizontal scan during abrupt changes of the intensity signal (referred to herein also as "spot arrest systems"). For example, U.S. Pat. No. 2,678,964 shows a system in which the horizontal scan is velocity modulated with "image enhancement signals" produced by taking various derivatives of the image intensity signal. The basic idea is to stop or slow the scan movement of the electron beam during the brightness transition period to allow time for the intensity control circuitry to change the brightness level at an image transition.
Historically, spot arrest systems have derived image enhancement signals from derivatives of the video signal because of the simplicity of implementation and the guaranteed correlation of the enhancement signal to transitions (as opposed to generating a fixed image enhancement signal for every transition). Such a system produces image enhancement signals which are proportional to the derivatives of the video signal and therefore proportional to the magnitude of the video transitions. Generally, image enhancement of a video picture is perceptible only when there is substantial beam retardation, roughly 75% or more from the nominal scan speed. Further, most transitions in typical commercial broadcast television are 50% or less than the maximum black to white transition. As a result, except during high level transitions, the degree of image enhancement (beam retardation or acceleration) in such typical prior art spot arrest systems using essentially first derivative image enhancement signals is usually imperceptible.
The problem was recognized and addressed by Lowry, U.S. Pat. No. 3,752,916. Lowry modified the image enhancement signal by a non-linear amplifier so that at a transition point of over approximately 10%, the image enhancement signal amplitude tended toward an optimum amplitude. At about the 10% transition level, the image enhancement signal amplitude was about 50% of the optimum amplitude. The enhancement fell off rapidly below 10%. While this falling off toward zero is advantageous, systems using this approach nevertheless produced an intolerable amount of noise which was manifested as intensity variations in the video display due to a velocity stuttered scan.
An investigatory approach by others at Zenith involved automatic gain adjustment of an image enhancement signal. A control signal was used to change the gain of an amplifier to generate a substantially constant amplitude enhancement signal. The control signal was based on the video signal and comprised a rectified first derivative signal which was then shaped to remove ripples in the top of the pulse. If the control signal was small in amplitude, the gain of the controlled amplifier was increased. Conversely, if the control signal amplitude was small, the amplifier gain was reduced. While this system did improve enhancement of some video transitions, it significantly increased noise.
One approach to removing noise in a particular part of a television system is disclosed in Morrison, U.S. Pat. No. 3,995,108. Morrison does not address spot arrest systems, but deals with aperture correction wherein luminance transitions are sharpened by increasing the amplitude of higher frequency components with respect to low frequency components. In such systems noise is amplified also. Morrison describes a system for removing such noise from aperture correction signals by using a gated coring system which passes or rejects signals on the basis of a control signal which corresponds to noise signals.
In addition to these problems of low transition level enhancement and excessive noise in spot arrest systems, a further problem is a change in edge contrast, that is, excessive brightness changes at the video transitions. There have been recent investigative efforts by others also at Zenith to compensate for brightness changes through a correction signal applied to the displayed luminance signal. In one such experimental effort, a control signal was multiplied with the luminance signal so that when brightness was in a decreasing state, the electron beam current was in an increasing state, and vice-versa. While a spot arrest system with such brightness compensation was satisfactory from the point of view of edge contrast, it was not able to enhance images satisfactorily over a sufficiently wide range of video transition levels.
Another problem with prior spot arrest systems is geometric distortion of the displayed images. For example, white objects may appear narrower, and black objects wider, than intended. Still further problems are, first, the undesirable effect of an artificial looking display and, second, the high cost of the systems.
Accordingly, it is a general object of the present invention to provide an improved method and apparatus for enhancing the sharpness of television images.
Another object is to provide a spot arresting system which does not suffer from excessive noise.
A further object is to provide such a system which provides visually perceptible edge enhancement over a wide range of video transition levels.